1. Field of the Invention
The present invention relates to a functional device subject to hermetical sealing, a semiconductor device including the functional device, and an electronic device including the functional device.
2. Description of the Related Art
As the integration technology has been improved in these years, a small and light-weight electronic device operating with a low voltage, a low electrical consumption, and a high frequency has been rapidly developed. Meanwhile, the electronic device needs the higher functions. As one of techniques to solve the foregoing tasks opposite to each other, MEMS (Micro Electro Mechanical System: micromachine) has attracted attentions. The MEMS is a system in which a micromechanical element and an electronic circuit element are fused by the silicon processing technique. The elemental technology of the MEMS has the superior characteristics such as the high precision processing. Therefore, the MEMS can realize a small and inexpensive SoC (System on a Chip) with the high functions.
Accordingly, the elemental technology of the MEMS is used in various application fields such as various sensors, fluid, optics, RF, storage, and biotechnology. As an MEMS device using such a technology, for example, a mixer for a communication circuit can be cited. The mixer has a movable portion using mechanical resonance on a substrate. Resonance is made in a frequency (intermediate frequency) as a difference between a frequency of an RF signal and a frequency of an LO signal. The signal of the intermediate frequency is outputted as an IF signal.
In a functional device including a functional portion such as the foregoing movable portion using the elemental technology of the MEMS described above, there is a possibility that the characteristics of the device are changed due to influence of an external environment such as temperature, pressure, moisture in the air, and dust, leading to lowering of the reliability of the device. In particular, in the step of mounting the MEMS device, the MEMS device is exposed to an environment severer than that in usage. Therefore, in some cases, the characteristics of the device are largely deteriorated, and the yield is lowered. In the case that mechanical resonance is used for operation of the functional portion, in some cases, the device is desirably operated under a reduced pressure in view of the characteristics. For the reasons mentioned above, in the MEMS device, to secure the mechanical operation space of the functional portion and maintain a stable environment, the functional portion needs to be hermetically sealed.
In the past, for hermetically sealing the functional portion, a method to directly cover the functional portion by bonding an airtight package has been employed. In this method, however, the manufacturing cost is high. Therefore, in recent years, many methods to hermetically seal the functional portion at the wafer level have been suggested. For example, the following method can be cited. A sealing layer is formed on the surface of a sacrifice layer covering the entire functional portion. After an aperture is formed in part of the sealing layer, the sacrifice layer is removed by selective etching through the aperture. Finally, the aperture is sealed by using a film forming method. Thereby, the functional portion is hermetically sealed. Further, in Japanese Unexamined Patent Publication No. 2002-94328, the following method is disclosed. The functional portion is arranged in a concave portion of a substrate, and an electrode spreading as an umbrella is formed in the upper part of the functional portion, a void (aperture) is provided between the substrate and the electrode, and the aperture is sealed by sputtering method. Thereby, the functional portion is hermetically sealed.